9,596 research outputs found
Analogue model for quantum gravity phenomenology
So called "analogue models" use condensed matter systems (typically
hydrodynamic) to set up an "effective metric" and to model curved-space quantum
field theory in a physical system where all the microscopic degrees of freedom
are well understood. Known analogue models typically lead to massless minimally
coupled scalar fields. We present an extended "analogue space-time" programme
by investigating a condensed-matter system - in and beyond the hydrodynamic
limit - that is in principle capable of simulating the massive Klein-Gordon
equation in curved spacetime. Since many elementary particles have mass, this
is an essential step in building realistic analogue models, and an essential
first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. Furthermore our model suggests
constraints on quantum gravity phenomenology in terms of the "naturalness
problem" and "universality issue".Comment: Talk given at 7th Workshop on Quantum Field Theory Under the
Influence of External Conditions (QFEXT 05), Barcelona, Catalonia, Spain, 5-9
Sep 200
Tolman wormholes violate the strong energy condition
For an arbitrary Tolman wormhole, unconstrained by symmetry, we shall define
the bounce in terms of a three-dimensional edgeless achronal spacelike
hypersurface of minimal volume. (Zero trace for the extrinsic curvature plus a
"flare-out" condition.) This enables us to severely constrain the geometry of
spacetime at and near the bounce and to derive general theorems regarding
violations of the energy conditions--theorems that do not involve geodesic
averaging but nevertheless apply to situations much more general than the
highly symmetric FRW-based subclass of Tolman wormholes. [For example: even
under the mildest of hypotheses, the strong energy condition (SEC) must be
violated.] Alternatively, one can dispense with the minimal volume condition
and define a generic bounce entirely in terms of the motion of test particles
(future-pointing timelike geodesics), by looking at the expansion of their
timelike geodesic congruences. One re-confirms that the SEC must be violated at
or near the bounce. In contrast, it is easy to arrange for all the other
standard energy conditions to be satisfied.Comment: 8 pages, ReV-TeX 3.
Sonoluminescence and the QED vacuum
In this talk I shall describe an extension of the quantum-vacuum approach to
sonoluminescence proposed several years ago by J.Schwinger. We shall first
consider a model calculation based on Bogolubov coefficients relating the QED
vacuum in the presence of an expanded bubble to that in the presence of a
collapsed bubble. In this way we shall derive an estimate for the spectrum and
total energy emitted. This latter will be shown to be proportional to the
volume of space over which the refractive index changes, as Schwinger
predicted. After this preliminary check we shall deal with the physical
constraints that any viable dynamical model for SL has to satisfy in order to
fit the experimental data. We shall emphasize the importance of the timescale
of the change in refractive index. This discussion will led us to propose a
somewhat different version of dynamical Casimir effect in which the change in
volume of the bubble is no longer the only source for the change in the
refractive index.Comment: 15 pages, 1 figure, uses sprocl.sty. Talk at the 4th Workshop on
Quantum Field Theory Under the Influence of External Conditions, Leipzig,
14-18 September, 199
Gravitational vacuum polarization III: Energy conditions in the (1+1) Schwarzschild spacetime
Building on a pair of earlier papers, I investigate the various point-wise
and averaged energy conditions for the quantum stress-energy tensor
corresponding to a conformally-coupled massless scalar field in the in the
(1+1)-dimensional Schwarzschild spacetime. Because the stress-energy tensors
are analytically known, I can get exact results for the Hartle--Hawking,
Boulware, and Unruh vacua. This exactly solvable model serves as a useful
sanity check on my (3+1)-dimensional investigations wherein I had to resort to
a mixture of analytic approximations and numerical techniques. Key results in
(1+1) dimensions are: (1) NEC is satisfied outside the event horizon for the
Hartle--Hawking vacuum, and violated for the Boulware and Unruh vacua. (2) DEC
is violated everywhere in the spacetime (for any quantum state, not just the
standard vacuum states).Comment: 7 pages, ReV_Te
Evaporation induced traversability of the Einstein--Rosen wormhole
Suppose, the Universe comes into existence (as classical spacetime) already
with an empty spherically symmetric macroscopic wormhole present in it.
Classically the wormhole would evolve into a part of the Schwarzschild space
and thus would not allow any signal to traverse it. I consider semiclassical
corrections to that picture and build a model of an evaporating wormhole. The
model is based on the assumption that the vacuum polarization and its
backreaction on the geometry of the wormhole are weak. The lack of information
about the era preceding the emergence of the wormhole results in appearance of
three parameters which -- along with the initial mass -- determine the
evolution of the wormhole. For some values of these parameters the wormhole
turns out to be long-lived enough to be traversed and to transform into a time
machine.Comment: v.2 A bit of discussion has been added and a few references v.3
Insignificant changes to match the published versio
Geometric structure of the generic static traversable wormhole throat
Traversable wormholes have traditionally been viewed as intrinsically
topological entities in some multiply connected spacetime. Here, we show that
topology is too limited a tool to accurately characterize a generic traversable
wormhole: in general one needs geometric information to detect the presence of
a wormhole, or more precisely to locate the wormhole throat. For an arbitrary
static spacetime we shall define the wormhole throat in terms of a
2-dimensional constant-time hypersurface of minimal area. (Zero trace for the
extrinsic curvature plus a "flare-out" condition.) This enables us to severely
constrain the geometry of spacetime at the wormhole throat and to derive
generalized theorems regarding violations of the energy conditions-theorems
that do not involve geodesic averaging but nevertheless apply to situations
much more general than the spherically symmetric Morris-Thorne traversable
wormhole. [For example: the null energy condition (NEC), when suitably weighted
and integrated over the wormhole throat, must be violated.] The major technical
limitation of the current approach is that we work in a static spacetime-this
is already a quite rich and complicated system.Comment: 25 pages; plain LaTeX; uses epsf.sty (four encapsulated postscript
figures
The causal structure of spacetime is a parameterized Randers geometry
There is a by now well-established isomorphism between stationary
4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries -
these Randers geometries being a particular case of the more general class of
3-dimensional Finsler geometries. We point out that in stably causal
spacetimes, by using the (time-dependent) ADM decomposition, this result can be
extended to general non-stationary spacetimes - the causal structure (conformal
structure) of the full spacetime is completely encoded in a parameterized
(time-dependent) class of Randers spaces, which can then be used to define a
Fermat principle, and also to reconstruct the null cones and causal structure.Comment: 8 page
Gravitational vacuum polarization IV: Energy conditions in the Unruh vacuum
Building on a series of earlier papers [gr-qc/9604007, gr-qc/9604008,
gr-qc/9604009], I investigate the various point-wise and averaged energy
conditions in the Unruh vacuum. I consider the quantum stress-energy tensor
corresponding to a conformally coupled massless scalar field, work in the
test-field limit, restrict attention to the Schwarzschild geometry, and invoke
a mixture of analytical and numerical techniques. I construct a semi-analytic
model for the stress-energy tensor that globally reproduces all known numerical
results to within 0.8%, and satisfies all known analytic features of the
stress-energy tensor. I show that in the Unruh vacuum (1) all standard
point-wise energy conditions are violated throughout the exterior region--all
the way from spatial infinity down to the event horizon, and (2) the averaged
null energy condition is violated on all outgoing radial null geodesics. In a
pair of appendices I indicate general strategy for constructing semi-analytic
models for the stress-energy tensor in the Hartle-Hawking and Boulware states,
and show that the Page approximation is in a certain sense the minimal ansatz
compatible with general properties of the stress-energy in the Hartle-Hawking
state.Comment: 40 pages; plain LaTeX; uses epsf.sty (ten encapsulated postscript
figures); two tables (table and tabular environments). Should successfully
compile under both LaTeX 209 and the 209 compatibility mode of LaTeX2
Closed Timelike Curves in Relativistic Computation
In this paper, we investigate the possibility of using closed timelike curves
(CTCs) in relativistic hypercomputation. We introduce a wormhole based
hypercomputation scenario which is free from the common worries, such as the
blueshift problem. We also discuss the physical reasonability of our scenario,
and why we cannot simply ignore the possibility of the existence of spacetimes
containing CTCs.Comment: 17 pages, 5 figure
The Hubble series: Convergence properties and redshift variables
In cosmography, cosmokinetics, and cosmology it is quite common to encounter
physical quantities expanded as a Taylor series in the cosmological redshift z.
Perhaps the most well-known exemplar of this phenomenon is the Hubble relation
between distance and redshift. However, we now have considerable high-z data
available, for instance we have supernova data at least back to redshift
z=1.75. This opens up the theoretical question as to whether or not the Hubble
series (or more generally any series expansion based on the z-redshift)
actually converges for large redshift? Based on a combination of mathematical
and physical reasoning, we argue that the radius of convergence of any series
expansion in z is less than or equal to 1, and that z-based expansions must
break down for z>1, corresponding to a universe less than half its current
size.
Furthermore, we shall argue on theoretical grounds for the utility of an
improved parameterization y=z/(1+z). In terms of the y-redshift we again argue
that the radius of convergence of any series expansion in y is less than or
equal to 1, so that y-based expansions are likely to be good all the way back
to the big bang y=1, but that y-based expansions must break down for y<-1, now
corresponding to a universe more than twice its current size.Comment: 15 pages, 2 figures, accepted for publication in Classical and
Quantum Gravit
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